The present assignee has previously invented a flat light sheet formed by printing microscopic vertical LED dice over a conductor layer on a flexible substrate to electrically contact the LED's bottom electrodes, then printing a thin dielectric layer over the conductor layer which exposes the LED's top electrodes, then printing a transparent conductor layer to contact the LED's top electrodes to connect them in parallel. The LEDs may be printed to have a large percentage of the LEDs with the same orientation so the light sheet may be driven with a DC voltage. The light sheet may have a thickness between 5-13 mils, which is on the order of the thickness of a sheet of paper or cloth.
FIGS. 1 and 2 illustrates such a light sheet 10. The shape of the light sheet 10 and the pattern of printed LEDs may be customized for a particular application.
In FIG. 1, a starting substrate 11 may be polycarbonate, PET (polyester), PMMA, Mylar or other type of polymer sheet, or even a thin metal film, paper, cloth, or other material. In one embodiment, the substrate 11 is about 25-50 microns thick.
A conductor layer 12 is then deposited over the substrate 11, such as by printing. The substrate 11 and/or conductor layer 12 may be reflective if the light from the LEDs is to only be emitted from the opposite side. For example, the conductor layer 12 may be a printed aluminum layer or a laminated aluminum film. Alternatively, a reflective layer may be first laminated over the substrate 11 followed by printing a transparent conductor layer 12 over the reflective film. A reflective film, including a white diffusing paint, may also be provided on the back surface of the substrate 11.
A monolayer of microscopic inorganic LEDs 14 is then printed over the conductor layer 12. The LEDs 14 are vertical LEDs and include standard semiconductor GaN layers, including an n-layer, and active layer, and a p-layer. GaN LEDs typically emit blue light. The LEDs 14, however, may be any type of LED emitting red, green, yellow, or other color light.
The GaN-based micro-LEDs are less than a third the diameter of a human hair and less than a tenth as high, rendering them essentially invisible to the naked eye when the LEDs are sparsely spread across the substrate 11 to be illuminated. This attribute permits construction of a nearly or partially transparent light-generating layer made with micro-LEDs. In one embodiment, the LEDs 14 have a diameter less than 50 microns and a height less than 10 microns. The number of micro-LED devices per unit area may be freely adjusted when applying the micro-LEDs to the substrate 11. A well dispersed random distribution across the surface can produce nearly any desirable surface brightness. Lamps well in excess of 10,000 cd/m2 have been demonstrated by the assignee. The LEDs may be printed as an ink using screen printing or other forms of printing. Further detail of forming a light source by printing microscopic vertical LEDs, and controlling their orientation on a substrate, can be found in US application publication US 2012/0164796, entitled, Method of Manufacturing a Printable Composition of Liquid or Gel Suspension of Diodes, assigned to the present assignee and incorporated herein by reference.
In one embodiment, an LED wafer, containing many thousands of vertical LEDs, is fabricated so that the top metal electrode 16 for each LED is small to allow light to exit the top surface of the LEDs. The bottom metal electrode 18 is reflective (a mirror) and should have a reflectivity of over 90% for visible light. There is some side light, depending on the thickness of the LED. In the example, the anode electrode is on top and the cathode electrode is on the bottom.
The LEDs are completely formed on the wafer, including the anode and cathode metallizations, by using one or more carrier wafers during the processing and removing the growth substrate to gain access to both LED surfaces for metallization. The LED wafer is bonded to the carrier wafer using a dissolvable bonding adhesive. After the LEDs are formed on the wafer, trenches are photolithographically defined and etched in the front surface of the wafer around each LED, to a depth equal to the bottom electrode, so that each LED has a diameter of less than 50 microns and a thickness of about 4-8 microns, making them essentially invisible to the naked eye. A preferred shape of each LED is hexagonal. The trench etch exposes the underlying wafer bonding adhesive. The bonding adhesive is then dissolved in a solution to release the LEDs from the carrier wafer. Singulation may instead be performed by thinning the back surface of the wafer until the LEDs are singulated. The LEDs 14 of FIG. 1 result, depending on the metallization designs. The microscopic LEDs 14 are then uniformly infused in a solvent, including a viscosity-modifying polymer resin, to form an LED ink for printing, such as screen printing, or flexographic printing.
The LED ink is then printed over the conductor layer 12. The orientation of the LEDs 14 can be controlled by providing a relatively tall top electrode 16 (e.g., the anode electrode), so that the top electrode 16 orients upward by taking the fluid path of least resistance through the solvent after printing. The anode and cathode surfaces may be opposite to those shown. The LED ink is heated (cured) to evaporate the solvent. After curing, the LEDs remain attached to the underlying conductor layer 12 with a small amount of residual resin that was dissolved in the LED ink as a viscosity modifier. The adhesive properties of the resin and the decrease in volume of resin underneath the LEDs 14 during curing press the bottom cathode electrode 18 against the underlying conductor layer 12, creating a good electrical connection. Over 90% like orientation has been achieved, although satisfactory performance may be achieved with over 75% of the LEDs being in the same orientation.
A transparent polymer dielectric layer 19 is then selectively printed over the conductor layer 12 to encapsulate the sides of the LEDs 14 and further secure them in position. The ink used to form the dielectric layer 19 pulls back from the upper surface of the LEDs 14, or de-wets from the top of the LEDs 14, during curing to expose the top electrodes 16. If any dielectric remains over the LEDs 14, a blanket etch step may be performed to expose the top electrodes 16.
A transparent conductor layer 20 is then printed to contact the top electrodes 16. The conductor layer 20 is cured by lamps to create good electrical contact to the electrodes 16.
The LEDs 14 in the monolayer, within a defined area, are connected in parallel by the conductor layers 12/20 since the LEDs 14 have the same orientation. Since the LEDs 14 are connected in parallel, the driving voltage must approximately equal the voltage drop of a single LED 14.
A flexible, polymer protective layer 22 may be printed over the transparent conductor layer 20. If wavelength conversion is desired, a phosphor layer may be printed over the surface. The phosphor layer may comprise phosphor powder (e.g. a YAG phosphor) in a transparent flexible binder, such as a resin or silicone.
The flexible light sheet 10 of FIG. 1 may be any size and may even be a continuous sheet formed during a roll-to-roll process that is later stamped out for a particular application.
FIGS. 1 and 2 also illustrate how the thin conductor layers 12 and 20 on the light sheet 10 may be electrically contacted along their edges by metal bus bars 24-27 that are printed and cured to electrically contact the conductor layers 12 and 20. The metal bus bars along opposite edges are shorted together by a printed metal portion outside of the cross-section. The structure may have one or more conductive vias 30 and 32 (metal filled through-holes), which form a bottom anode lead 34 and a bottom cathode lead 36 so that all electrical connections may be made from the bottom of the substrate 11. Instead of vias, the top metal may be connected to the bottom metal by other means, such as metal straps extending over the edges of the light sheet. A suitable voltage differential applied to the leads 34 and 36 turns on the LEDs 14 to emit light through one or both surfaces of the light sheet 10.
FIG. 2 is a top down view of the light sheet 10 of FIG. 1, where FIG. 1 is taken along line 1-1 in FIG. 2. If the light sheet 10 is wide, there will be a significant IR drop across at least the transparent conductor layer 20. Thin metal runners 38 may be printed along the surface of the conductor layer 20 between the opposing bus bars 24 and 25 to cause the conductor layer 20 to have a more uniform voltage, resulting in more uniform current spreading. In an actual embodiment, there may be thousands of LEDs 14 in a light sheet 10.
One problem encountered with the light sheet 10 is that the various interfaces of different indices of refraction, such as the interface of air and the top polymer layer 22, are flat and parallel and internally reflect light when the incident light ray is greater than the critical angle. This creates waveguiding, where light is trapped in the flat layers and eventually absorbed. Two such light rays 40 and 42 are shown. Light ray 43 shows an internal reflection off the bottom surface of the layer 22. With each reflection, the light becomes absorbed somewhat, leading to a decrease in light extraction efficiency.
What is needed is a technique for increasing the light extraction efficiency of the printed structure of FIGS. 1 and 2 that is compatible with the existing fabrication process.